Bringing down the cost of PERSIST construction
I really like the idea of PERSIST houses. Keeping insulation on the exterior makes a lot of sense, and there are few more foolproof methods to build a strong air barrier. But the undeniable drawback to PERSIST construction is cost. Peel and stick membranes are significantly more expensive than standard housewrap, and there is a lot of OSB sheathing involved. REMOTE constructions address some of these concerns, though really only work with unconditioned attic spaces. I was trying to think of ways to bring down cost and reduce material use, and I had an idea I wanted feedback on.
In short, the idea was to eliminate the peel-and-stick membrane and use exclusively foam sheathing, then spray a layer of foam between the studs, directly onto the rigid exterior foam — similar to how you would do a flash and batt construction. The spray foam would seal nearly all the gaps between the rigid foam boards, providing a strong air barrier. The hope is that eliminating the peel and stick membrane and OSB on exterior walls will bring down cost relative to the additional spray foam.
Obviously this approach would require an alternate method of shear-bracing the walls, likely 1×4 let-in bracing. Further, one of the layers of exterior rigid foam (likely the first) would need to be tape sealed to qualify as a WRB. The tape would be nicely reinforced by the spray foam, reducing the chance of the WRB being compromised, along with the air barrier. As an added benefit, without OSB, the consequences of water leakage are reduced.
Either open or closed cell spray foam could hypothetically be used between the studs, though closed cell might be marginally safer. Since the spray foam isn’t being relied on for insulation as much as air sealing, studs could be filled part way, leaving a service cavity. Additional layers of exterior foam are likely cheaper anyway. As per standard PERSIST methods, 2×4 framing would be used.
From what I can tell, the main weakness of the air barrier would be between floors on a multi-story building. However, this is common in a lot of air barrier methods, and there exist solutions to this problem that aren’t too costly. Even just lining up the rigid foam to eliminate cracks over this area should reduce air leakage to well within acceptable levels.
My main questions are:
Does this approach have any glaring mistakes?
Would this approach likely perform similarly to traditional PERSIST construction?
Would this method actually reduce cost relative to PERSIST construction? Would it be competitive with 2x6s with cellulose insulation and exterior rigid foam?
Is it likely to run afoul of code?
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